Communication systems on which data is transmitted are well known in the art. Before a user may transmit data, many systems require a data (communication) unit to first request a communication resource, such as a frequency, frequency pair, TDM (time division multiplexed) time slot, and so forth. The system, by way of a controller, receives the request, also known as a reservation request, allocates a communication resource, and transmits a reservation grant that provides the user with the exclusive use of the assigned communication resource during an allotted time.
Imperfect channel conditions found in a mobile radio data system can result in degradation of system performance. Channel conditions that can cause poor message reception include low signal-to-noise ratio, co-channel interference, multipath fading, and so forth. When a data unit fails to receive a message correctly, many systems allow the unit to request a retransmission of the message. Such requests for retransmission and the retransmissions themselves, which may be numerous, consume valuable communication resources, which is inefficient and wasteful in general.
Some wireline systems permit a range of data rates to be used in an attempt to optimize throughput by matching the data rate to the current wireline conditions. In other words, when a wireline channel has good quality conditions, a higher data rate is used, and when wireline channel conditions are poor, a lower data rate is transmitted. The lowest rate (offering the highest level of error protection) is very inefficient, although it has the highest likelihood of successful receipt. If the lowest data rate used under both good and poor signal conditions, transmission overall is inefficient in that a minimal amount of information is transmitted in a fixed time period. While use of a higher data rate for transmission is a more efficient use of wireline channel, such a data rate has the lowest probability of successful reception over all signal conditions, and consequently is not received with the greatest probability of successful decoding, resulting in the need to retransmit data messages. To solve the problem of high efficiency of channel versus high probability of receipt, many systems use a middle-of-the-road data rate, i.e., a rate in between the highest and lowest rates available in the system. Even with a medium data rate, data may not be transmitted reliably due to poor wireline conditions, and the error rate can be significant.
Another method for improving resource efficiency, not relying on modifying the transmission rate, is commonly referred to as an automatic repeat request (ARQ). In such a system, the receiving data unit detects which data frames are received in error and sends a feedback message to the transmitting end requesting that a particular frame, or group of frames, be re-transmitted. In this manner, the rate at which the data is sent over the channel is effectively increased or decreased in response to the channel conditions, and consequently, effective channel throughput changes in response to the channel conditions. Such a method, however, requires a significant amount of overhead, both in channel utilization and in transmission delay. In particular, when channel conditions are poor over an extended period of time, the need for re-transmission is high, and the channel utilization is correspondingly low.
Different techniques for modulating data can be used to achieve effective communication over a particular channel. Such modulation techniques might also incorporate error correction coding to improve performance under poor channel conditions. Advanced modulation techniques are generally more prone to incurring errors, and are likely to use error correction. Error correcting schemes typically increase the amount of overhead, thereby reducing the maximum rate at which data may be sent.
Accordingly, there is a need for a wireless data communication system that efficiently uses system resources over a wide range of channel conditions while limiting overhead that costs valuable bandwidth.